This invention relates to probes for magnetic resonance (MR) imaging apparatus.
It has recently been demonstrated that MR probes may be built into catheters, allowing diagnostically useful high resolution images to be obtained from within small, intravascular, structures.
These catheter probes have been either of the elongated-loop type, with high local sensitivity, (High Resolution Intravascular MRI and MRS by Using a Catheter Receiver Coil, by Ergin Atalar, Paul A Bottomley, Ogan Ocali, Luis C L Correia, Mark D Kelemen, Joao A C Lima and Elias A Zerhouni, Magn. Reson. Med. 36:596-605 (1996)), or of the loop-less antenna design with lower local sensitivity but superior field of view (Intravascular Magnetic Resonance Imaging Using a Loop-less Catheter Antenna, by Ogan Ocali and Ergin Atalar, Magn. Reson. Med. 37:112-118 (1997)). The former is primarily coupled to the magnetic field, the latter primarily coupled to the electric field.
FIG. 1 is an axial sectional view of the latter. The probe is intended to be inserted axially in the right-hand direction as seen in the drawing. The antenna is a dipole 1, 2, consisting of a hollow conductor 1, and a solid conductor 2, both of the same diameter. The feed is a coaxial cable indicated generally by the reference numeral 3, the centre conductor 4 of which connects to the arm 2, and the coaxial sheath 5 of which has a reduced diameter region 6 which connects to the right-hand end of the hollow arm 1 via a region 7. An insulator 8 separates the hollow arm 1 from the reduced diameter part 6 of the coaxial sheath 5 of the feed 3. Capacitance between these two would cause current to leak from the arm 1, so the dielectric constant of the insulator, in conjunction with the diameter of the parts 1, 6, is chosen so that the transmission line formed by the parts 1, 8, 6 is one quarter-wavelength in length, which creates a high impedance between the left-hand end of the arm 1 and the part 6, thus reducing leakage current. The length of the arm 2 is also one quarter-wavelength, that is, of the magnetic resonance frequency and in the material of the tissue in which it is to be used.
The invention provides a probe for magnetic resonance imaging apparatus, comprising a dipole, one arm of which includes an elongate receive loop.
The probe is capable of performing the role of loop-less dipole and of elongated receive loop. In one embodiment, the probe is capable of simultaneous imaging in both modes. By combining the signals from the two modes, the field of view of the probe might be increased, with the possibility of SNR gains close to the coil.